Organic materials have recently shown promise as the active layer in organic based thin film transistors and organic field effect transistors [see H. E. Katz, Z. Bao and S. L. Gilat, Acc. Chem. Res., 2001, 34, 5, 359]. Such devices have potential applications in smart cards, security tags and the switching element in flat panel displays. Organic materials are envisaged to have substantial cost advantages over their silicon analogues if they can be deposited from solution, as this enables a fast, large-area fabrication route.
The performance of the device is principally based upon the charge carrier mobility of the semi-conducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with a high charge carrier mobility (>1×10−3 cm2 V−1 s−1). In addition, it is important that the semi-conducting material is relatively stable to oxidation i.e. it has a high ionisation potential, as oxidation leads to reduced device performance.
Regioregular head-to-tail poly(3-hexylthiophene) has been reported with charge carrier mobility between 1×10−5 and 4.5×10−2 cm2 V−1 s−1, but with a rather low current on/off ratio between 10 and 103 [see Z. Bao et al., Appl. Pys. Lett., 1996, 69, 4108]. This low on/off current is due in part to the low ionisation potential of the polymer, which can lead to oxygen doping of the polymer under ambient conditions, and a subsequent high off current [see H. Sirringhaus et al., Adv. Solid State Phys., 1999, 39, 101].
A high regioregularity leads to improved packing and optimised microstructure, leading to improved charge carrier mobility [see H. Sirringhaus et al., Science, 1998, 280, 1741–1744; H. Sirringhaus et al., Nature, 1999, 401, 685–688; and H. Sirringhaus, et al., Synthetic Metals, 2000, 111–112, 129–132]. In general, poly(3-alkylthiophenes) show improved solubility and are able to be solution processed to fabricate large area films. However, poly(3-alkylthiophenes) have relatively low ionisation potentials and are susceptible to doping in air.
It is an aim of the present invention to provide new materials for use as semiconductors or charge transport materials, which are easy to synthesize, have high charge mobility, good processibility and oxidative stability.
Another aim of the invention is to provide new semiconductor and charge transport components, and new and improved electrooptical, electronic and electroluminescent devices comprising these components, like field effect transistors (FET) as components of integrated circuitry or of thin film transistors (TFT), and organic light emitting diode (OLED) applications like electroluminescent displays or backlights of liquid crystal displays.
Other aims of the invention are immediately evident to those skilled in the art from the following description.
The inventors have found that these aims can be achieved by providing mono-, oligo- and (co)polymers of thieno[2,3-b]thiophene as semiconductors and charge transport materials.
The inventors of this invention have found that incorporation of thieno[2,3-b]thiophene (1) units into conjugated polymers yields materials which are useful for charge transport in FET's, and for electroluminescence in OLED's. The incorporation of this core has the effect of lowering the ionisation potential of the resulting polymers, resulting in improved air stability and reduced transistor off currents. It is believed that this effect is due to the fact that quinoidal type resonance structures cannot be formed for thieno[2,3-b]thiophene as shown in Diagram 1, which therefore limits the effective conjugation length of the polymer backbone, since charge cannot delocalise through this unit. This is in contrast for the other regioisomer, thieno[3,2-b]thiophene (2), or 2,2-bithiophene (3), in which fully delocalised quinoidal-type structures can be realised, as shown in Diagram 1.
Diagram 1: Charge delocalisation though dithiophene units, where A represents a conjugated species such as a thiophene or benzene ring.

Therefore once an exciton or hole is formed in the polymer, it cannot fully delocalise along the polymer backbone, as depicted in Diagram 2. This limits the effective conjugation to only the aromatic units between any two thieno[2,3-b]thiophenes (in Diagram 2, effectively 2 thiophene units plus 2 thieno[2,3-b]thiophene units). This provides a ready means to tune the effective conjugation length in a conjugated polymer. This is desirable because shorter conjugation lengths result in lower HOMO energy levels (since there are less degenerate states) and therefore improved oxidative stability. Since the dominant mechanism for charge transport in conjugated polymers is via a hole-hopping mechanism, the polymers are still able to exhibit good mobilities providing close packed morphology is obtained.
Diagram 2: Effective conjugation in a co-polymer containing thieno[2,3-b]thiophene.

In electroluminescent applications this provides a ready means to colour tune the polymers, and also results in improved colour purity since each polymer chain now has the same effective conjugation length.
Furthermore, in contrast to other aromatic units that cannot form quinoidal type delocalisation, such as meta-substituted benzenes, or 2,7-substituted naphthalene's, thieno[2,3-b]thiophene has a linear shape and does not introduce ‘kinks’ in the polymer backbone. Such ‘kinks’ can result in amorphous polymers.
G. Koβmehl et al., Makromol. Chem. 1982, 183, 2747–2769 disclose poly(thieno[2,3-b]thiophene-2,5-divinylenearylene), wherein the arylene group is 2,5-thiophene, 1,4-phenylene or 2,5-dimethoxy-1,4-phenylene. However, copolymers of this type with a vinyl linker do often show stability problems, and copolymers comprising unsubstituted or methoxy substituted phenylene groups often have low solubility.